1. Field of the Invention
The present invention relates to decoding systems for multiple transport streams, such as MPEG-2 (an international standard originated by the Moving Picture Experts Group) encoded data steams. In its particular respects, the present invention is directed to extraction of program specific information (PSI) from the multiple transport streams, to enable a so-called "channel change".
2. Description of the Related Art
As described in ISO/IEC 13818, a decoding system consisting of an MPEG-2 transport decoder, and elementary stream decoders including at least one MPEG-2 audio decoder and MPEG-2 video decoder may receive MPEG-2 transport streams (bitstreams) from multiple source types and/or multiple service providers. Any one MPEG-2 transport stream may contain multiple programs for presentation to the user. Thus, the action of switching from one program (or "channel") to another (usually described as a "channel change") may or may not necessitate switching from one transport stream source to another.
In the case where the data for the desired new channel is in the transport stream currently being received by the system decoder, the latency between selecting a new channel and displaying the new data to the user is limited only by the ability of the transport decoder and elementary stream decoders to resynchronize to the new data (which can be on a new time base as described in the ISO/IEC 13818 document). This latency should be rather small.
However, where a channel change requires selecting data that is not in the currently used transport stream, but in another transport stream, the latency is much greater. In such a case, the transport stream being fed to the transport decoder must be switched to a new transport stream. When this is done, the transport decoder cannot begin to parse and decode the program from the new transport stream until it has recovered program specific information (PSI) tables from the new stream. Since these tables are not sent frequently in any one transport stream, there can be a large delay in time before PSI table information arrives at the transport decoder for extraction. During this delay, no other decoding can be performed since basic information (such as where data is stored in the new stream, and what type of data is present) about the stream is not available. Thus, a large channel change delay is incurred, which may be long enough to become annoying to the user.